Old dog has barking problem

[Sorgoth]

I have a miniature Shnauzer who recently turned nine.

In his younger days, we...well, we didn't bother training him that well, and now he's had a barking problem for a long time, but it seems like it's gotten worse towards strangers lately.

We tried a shock collar, and he figured out how to push it with his paws so that it didn't shock him anymore...thought it was a mistake first time, put it back on right, repeated multiple times...

I've tried just ignoring him, holding his muzzle shut, etc.

What can I do? Is it too late?